Commercial aircraft fly at very high altitudes. This is because, at least in part, the relatively thinner air at higher altitudes reduces drag. As a result, modern commercial aircraft fly in the range of approximately 30,000-45,000 feet above sea level (ASL), while private jets may fly as high as approximately 51,000 feet ASL. And, while flying at this altitude increases efficiencies, it also requires that some technical difficulties to be overcome.
One of these difficulties is that the air at these altitudes does not contain enough oxygen to sustain human life. At altitudes above approximately 12,000-14,000 feet ASL, most humans begin to suffer from altitude induced hypoxia. Early solutions to this problem included oxygen mask systems for providing additional oxygen directly to users. Such systems are still used in fighter aircraft, for example, where oxygen need only be provided for one or two users. In large commercial aircraft, however, oxygen mask systems are impractical due to, for example, the number of passengers, size and space requirements, and the need for passengers and flight attendants to be able to move about the cabin.
A more practical solution to this problem is to pressurize the cabin. This enables the cabin to simulate conditions at lower altitudes (generally, similar conditions to those found at somewhere between 4,500 and 8,000 feet ASL). This feature was introduced in 1938 on the Boeing 307—the first commercial airliner with a pressurized cabin.
Conventionally, pressurized and conditioned air is supplied into the cabin and is provided by, for example, bleed air from the compressor side of a turbine engine. The pressure in the cabin is then controlled using a Cabin Pressure Outflow Valve (CPOV). Because the atmospheric pressure decreases with altitude, the pressure differential between the pressurized interior of the cabin and the atmosphere increases with altitude. This results in subsonic flow through the CPOV at lower pressure differentials. Ideally, for maximum thrust recovery, the CPOV gate surfaces will be smooth. However, this can result in flow separation, which can result in tonal noise. At low differential pressures, the tonal noise can propagate into the airplane cabin. At higher altitudes, however, the increased pressure differential increases the velocity of the flow, often to supersonic levels, which prevents the tonal noise from entering the airplane cabin.
Conventional CPOVs often have fixed aerodynamic devices. These devices in various combinations can be used to prevent flow separation, for example, to reduce tonal noise at lower differential pressures (e.g., during take-off, climb, descent, and landing). The tonal noises (e.g., whistles or whines) created during these flight regimes are particularly bothersome to passengers and crew and should be eliminated, to the extent possible, in commercial aircraft.
These fixed aerodynamic devices, however, can produce additional broadband noise at lower and higher differential pressures (e.g., during the full flight regime) and also represent a possible flow inefficiency through the CPOV. As a result, while these devices are deployed at all times (i.e., because they are molded into the gate, for example), they are only needed in fairly limited conditions (i.e., only during fairly small portions of the flight regime). The tonal noise at lower differential pressures, for example, is transitory; yet, the fixed aerodynamic devices can result in increased broadband noise even when no tonal noise is present. Thus, implementing retractable aerodynamic devices will result in reduced broadband noise and improved thrust recovery for a large portion of the flight regime.
It is with such considerations in mind that embodiments of valves for pressurized aircraft cabins are described.